The invention relates to methods and apparatuses for direct conversion of uranium hexafluoride (UF.sub.6) into uranium oxide (UxOy).
Dry direct conversion of UF6 into uranium oxide has been in industrial use for a long time. Apparatuses for implementing it generally comprise the following components, in succession:
a reactor provided with injection means for injecting UF6 and steam so as to cause UO.sub.2 F.sub.2 to be formed by hydrolysis; PA1 a rotary tubular pyrohydrolysis oven for transforming UO2F2 into uranium oxide, the furnace being provided with heater means and with injection means for injecting a counterflow of steam and hydrogen; and PA1 means for conditioning the oxide powder. PA1 a reactor provided with injection means for injecting UF.sub.6 and steam, so as to cause UO2F2 to be formed by hydrolysis; and PA1 a rotary tubular pyrohydrolysis furnace for transforming UO.sub.2 F.sub.2 into uranium oxide, the furnace being provided with heater means and with injection means for injecting a counterflow of steam and hydrogen.
The methods used until now have been based on the assumption that the reactions implemented are simple, and of the form: EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF in the reactor EQU UO.sub.2 F2+H.sub.2 O.fwdarw.UO.sub.3 +2HF and UO.sub.3 +H.sub.2 .fwdarw.UO.sub.2 +H.sub.2 O in the furnace.
Given the simplicity of those reactions, it was believed that individual temperature control in three successive zones of the rotary tubular furnace would suffice.
However, careful analysis of the reactions involved show that they are more complicated; some of them are endothermal while others are exothermal; their reaction kinetics are different.
In the oven or furnace there is initially a pyrohydrolysis reaction which is the sum of two reactions where the first is highly endothermal while the second is slightly exothermal: EQU UO.sub.2 F2+H.sub.2 O.fwdarw.UO.sub.3 +2HF EQU 3UO.sub.3 .fwdarw.U.sub.3 O.sub.9 +1/2O.sub.2
Those two reactions are not strictly separated. As UO3 forms, it tends to pass to the U.sub.3 O.sub.8 state with different kinetics. Overall the pair of reactions is highly endothermal.
The subsequent reaction whereby U.sub.3 O.sub.8 is reduced to UO2 by means of hydrogen also includes intermediate reactions that can be outlined by the following transformations: EQU UO.sub.2 .fwdarw.U.sub.3 O.sub.8 -x EQU U.sub.3 O.sub.8 -x.fwdarw.U.sub.4 O.sub.9 EQU U.sub.4 O.sub.9 .fwdarw.UO.sub.2
Each reaction has its own activation energy and its own enthalpy.
Those reactions take place in a counterflow of gas which is injected into the downstream end of the furnace and which initially forms a mixture of hydrogen and of steam which is generally admitted at a temperature of about 350.degree. C. As it flows (in a counterflow relative to powder), the mixture becomes charged with hydrofluoric acid produced by the reaction.
The tubular furnace must be provided with heater means. In the downstream portion of the furnace (in the powder flow direction), the heater means must raise the temperature of the gas to a value that is sufficient to cause reduction to UO.sub.2. At the inlet to the furnace, it is necessary to feed the powder and the gases at the pyrohydrolysis temperature.